feat(desktop): show volume and brightness changes with a SwayOSD popup

Route the volume and brightness keys through swayosd-client, which raises
a transient on-screen popup that auto-hides a moment after the last press,
so the level is visible while adjusting without a permanent widget. A new
osd module runs the server for the graphical session and makes the
backlight node group-writable so it dims without root.

Supersedes the plain brightnessctl binds, whose control swayosd bundles.

modules/desktop/desktop.nix

@@ -25,6 +25,6 @@ in
     modules.desktop.theming.enable = lib.mkDefault true;
     modules.desktop.waybar.enable = lib.mkDefault true;
     modules.desktop.audio.enable = lib.mkDefault true;
-    modules.desktop.brightness.enable = lib.mkDefault true;
+    modules.desktop.osd.enable = lib.mkDefault true;
   };
 }